M. Heydari-Malayeri - Paris Observatory

A → magnetohydrodynamic model devised to
account for the → bipolar jets and
→ outflows observed around → protostars.
Basically, a → poloidal magnetic field is frozen into a rotating
→ accretion disk.
If the angle between the magnetic field lines threading the
disk and the rotation axis of the disk is larger than 30°,
the plasma can be accelerated out of the accretion disk along the field lines.
The field lines rotate at a constant → angular velocity,
and as the gas moves outward along the
field lines, it is accelerated by an increasing → centrifugal force
(magnetocentrifugal acceleration). At some point, when the rotation
velocity is about the same as the → Alfven velocity
in the gas, the
field lines get increasingly wound up by the inertia of the attached
gas and a strong → toroidal magnetic field
component is generated. The toroidal
component is the main agent in collimating the flow into a direction
along the → open magnetic field lines. The earliest version of
the model was proposed by Blandford & Payne (1982, MNRAS 199, 883).
It has two main versions: → X-wind
and → disk wind models.
See also → magnetorotational instability.